The principle objective of this study, is to elucidate the kinetics of calcium metabolism in normal pre-pubertal children and to evaluate disease related changes in calcium metabolism in both children and adults. Stable isotopes make such studies possible in children and in women of child-bearing age, for whom the risk of radioactive calcium tracers would be unacceptable. Further, stable isotopic tracers simplify studies in populations for whom radio-tracers are an acceptable risk, and they permit repeated measurements. Isotope ratio mass spectrometry using a solid probe introduction technique and a quadruple mass filter are the methods used. Isotope ratio results are analyzed with a multi-compartmental mathematical model from which mineral mixing kinetics, and metabolic fluxes are determined. Studies now in progress in collaboration with HGB, NICHD evaluate calcium kinetics during, normal childhood growth and development, and in response to growth hormone, sex steroids, normal puberty, and vitamin D therapy. These will be among first data for calcium metabolism in normal children. Studies in collaboration with Dr. Laura Hillman, (University of Chicago) are being designed to evaluate children with genetic disorders. Studies with Dr. Irving Rosenberg are being designed to evaluate malabsorption, metabolic bone diseases and calcium metabolism during total parenteral nutrition. Results of preliminary studies in neonates and several studies of subjects with inborn errors of calcium metabolism are now ready for publication. A second major area involves study of glucose metabolism in subjects with glycogen storage disease, type I. Use of glucose labeled with non-radioactive carbon and/or deuterium will give access to a patient population hitherto unavailable for study. Specifically, deuterium labels at particular sites in the glucose molecule will be used to elucidate the kinetics of glucose metabolism, by comparison with the kinetics of a stable carbon label at a different site. High performance liquid chromatography (HPLC) thermospray mass spectrometry will be used for direct analysis of the labeled glucose molecules.